In recent years, wireless data communication in domestic and enterprise environments have become increasingly commonplace and an increasing number of wireless communication systems have been designed and deployed. In particular, the use of wireless networking has become prevalent and wireless network standards such as IEEE 801.11a and IEEE 801.11g have become commonplace.
The requirement for increasing data rates, communication capacity and quality of services has led to continued research and new techniques and standards being developed for wireless networking. One such standard is the IEEE 801.11n standard which is currently under development. IEEE 801.11n is expected to operate in the 2.4 GHz or 5 GHz frequency spectrum and promises data rates of around 100 Mbps and above on top of the MAC layer. IEEE 801.11n will use many techniques which are similar to the earlier developed IEEE 801.11a and IEEE 801.11g standards. The standard is to a large extent compatible with many of the characteristics of the earlier standards thereby allowing reuse of techniques and circuitry developed for these. For example, as in the previous standards IEEE 801.11a and IEEE 801.11g, IEEE 801.11n will use Orthogonal Frequency Division Multiplex (OFDM) modulation for transmission over the air interface.
Furthermore, in order to improve efficiency and to achieve the high data rates, IEEE 801.11n is planned to introduce a number of advanced techniques. For example, IEEE 801.11n communication is expected to typically be based on a plurality of transmit and receive antennas. Furthermore, rather than merely providing diversity from spatially separated transmit antennas, IEEE 801.11n will utilise transmitters having at least partially separate transmit circuitry for each antenna thus allowing different sub-signals to be transmitted from each of the antennas. The receivers may receive signals from a plurality of receive antennas and may perform a joint detection taking into account the number and individual characteristics associated with each of the plurality of transmit antennas and receive antennas. Specifically, IEEE 801.11n has seen the likely introduction of a Multiple-Transmit-Multiple-Receive (MTMR) antenna concept which exploits Multiple-Input-Multiple-Output (MIMO) channel properties to improve performance and throughput.
In order to enable or facilitate reception, the standards of IEEE 802.11a/g as well as all 802.11n proposals prescribe that all data packet are preceded by a physical layer preamble which comprise known data that facilitates receiver gain setting, synchronization and channel estimation. In addition, a dedicated OFDM symbol is included which conveys physical layer signaling required for the decoding of the data packet. This information includes, among others, information of the modulation scheme, coding rate and packet length for the data packet. This signaling is known as the SIG field. Since IEEE 802.11n receivers require information relating to multiple antennas, the signaling field has been enhanced for IEEE 802.11n and is generally referred to as SIG-N. The SIG-N fields are communicated as QPSK symbols in the subcarriers of the dedicated SIG-N field OFDM symbol.
Specifically, SIG-N mapping (QPSK) has been defined in the context of a proposal (Cenk Kose, Bruce Edwards, “WWiSE Proposal: High throughput extension to the 802.11 Standard”, IEEE document number 11-05-0149-02-000n) to IEEE802.11n.
In such systems, no signaling information is available prior to the transmission of the SIG-N field and the receiver must thus be able to decode this field without any prior information about its nature (this is necessary for compatibility reasons).
In addition to providing high data rate services, IEEE 802.11n is also expected to be used for a variety of applications having different requirements and characteristics. For example, IEEE 802.11n may be used for lower data rate applications, such as mobile Voice over Internet Protocol (VoIP) and mobile multimedia streaming for handheld devices. Although these applications have low data rate it is desired that they can be accessed over a large area and therefore it is desirable for IEEE 802.11n cells to have as large a coverage area as possible.
In order to extend the range of IEEE 802.11n cells, low data rate robust modes have been proposed which exploit the potential of the MIMO configuration to increase range.
Some of the proposed modes use a Space Time Block Code (STBC) combined with the low order constellation BPSK (Binary Phase Shift Keying) to provide a robust communication at lower signal to noise ratios. In other proposals, range extension is achieved by efficient use of beamforming techniques.
However, a problem for these robust modes is that the range extension applied to the user data may result in a range which exceeds that achieved for the OFDM symbols containing the SIG-N fields. In order to overcome this discrepancy, it has been proposed to introduce an optional simple repetition of the SIG-N field such that the OFDM symbol is transmitted twice.
This approach may provide a 3 dB signal to noise ratio improvement and may thereby increase the range. However, although this extension is beneficial it is not optimal and may in some situations limit the effective coverage area of the IEEE 802.11n cell.
Hence, an improved OFDM communication system would be advantageous and in particular a system allowing increased range, improved compatibility between user data and signalling transmissions, low complexity and/or improved performance would be advantageous.